A considerable attention from academies, governments and private institutions in many countries is being given to the development of biochips. In the field of deoxyribonucleic acid (DNA) chips, such as DNA sequences analyses, capillary electrophoreses, nucleic acid amplification, and parallel gene expression analyses are progressing toward maturity. Based on these developments, a series of emerging analytical methods are becoming available quickly, for instance, cell separation, cell-mediated immuno-assay, and high-throughput assays for new drug screening, which combined with combinatorial chemistry. The biochips are being fabricated by low cost plastic technology and elastomers, in addition to presently dominating silicon technology.
A unique trait of a biosensor is its combination with biological elements to form part of sensing mechanism, and by connecting with transducers to achieve the purpose of the detection of biological reactions; moreover, its integration with Micro-Electro-Mechanical Systems (MEMS) processes is named “bio-chip”. Among the related technologies of chip development, optical methods for detection offer high sensitivity and possible high throughput solutions, wherein fluorescence has been the chosen methods for many applications in the past. Surface plasmon resonance (SPR) becomes an emerging research tool for its non-labeling and real-time detection traits. The SPR biosensor exploits the optical principle of SPR as the transducer for biomolecule recognition. As the changes of composition, concentration or constituent within the sensing volume, it might result in a change of refractive index, and in turn a change in the resonant angle of SPR through the dissipated light energy. SPR, existing at the interface between a metal and a dielectric, is excited by a TM-polarized light beam, where the electrical field and magnetic field vectors are perpendicular and parallel to the plane of the interface respectively and are decaying exponentially on both directions. After the surface modification of the sensing area, protein molecules, e.g. antigens or antibodies, can be immobilized onto the activated surface for subsequently hybridized with corresponding molecules through recognition processes. Theoretically, only the bonded analytes, after washing, could influence the intensity of the reflected light. The substances, which are not within the sensing volume of surface plasmon wave, wouldn't affect the detecting outcome. Therefore, this method can offer very high discrimination of few hundred nanometers.
From the literature reviews, the state of art technology for SPR waveguide sensors is mainly on planar waveguide SPR sensors. The fabrication processes of such a waveguide sensor starts with a BK7 glass substrate, followed by thin film processes and etching to form a metallic pattern in a waveguide, and finally ion implantation into the substrate by high temperature ion exchange to change refractive index of the substrate. A layer of metallic film and a buffer layer of dielectric material are used for adjusting the sensing range, which are required for the waveguide to realize the SPR phenomenon. Another method is for optical-fiber type SPR sensors, which are made by stripping the cladding of the optical fiber and followed by a metal plating process.
There are two major categories of SPR waveguide detection schemes; one is by intensity modulation while the other is by wavelength modulation. The intensity type is the popular one and the earliest one, which satisfies the demands of higher intensity of single-wavelength laser beam to balance the loss of waveguides. The wavelength type is the latest one beneficial by the progress of the latest techniques in optical fiber, and its detection wouldn't need intense light for a moderate loss. An advantage of the wavelength type over the intensity type in detection is the wider dynamic range of the refractive index, and eliminates the constraint of narrow frequency bandwidth of the laser beam. In the curvature waveguide, it is known to have energy loss at the curvature location, so that the curvature radius has to be larger than the minimum curvature radius, which is subject to the refractive index difference of fiber core and sensing volume. It is actually depending on the experimental conditions.
The SPR sensing devices of the related arts use a glass slide as the substrate, where the majority is planar configuration, which is not handy for the detection by the portable instrument. The waveguide design is also by a straight-in-straight-out fashion, lacks of an optimum in size and also fails to offer a handy user interface.